Search: parse tags from a tags query parameter

[yvrhdn]

What this PR does:
Experiment to try out parsing search request tags from a tags query parameter.

Instead of specifying every tag as it's own query parameter, we put them all in the tags query parameter, separated by a space (logfmt style).
To remain compatible with the current Grafana search implementation we still parse top-level tag query parameters. As Grafana updates this can be removed.

The actual parsing of tags is handled by go-logfmt.

In a next PR I plan to move this code into pkg/api and reuse code with ParseBackendSearch (if it makes sense)

Which issue(s) this PR fixes:
Experiment related to #1038

Checklist

• Tests updated
• Documentation added --> docs: update search API #1062
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yvrhdn]

You can send a test request using curl:

curl -Gs http://localhost:3200/api/search --data-urlencode 'tags=service.name=tempo http.url=api/search' --data-urlencode minDuration=100ms

This will send the following request:

/api/search?tags=service.name%3Dtempo%20http.url%3Dapi%2Fsearch&minDuration=100ms

[mapno]

LGTM

[mdisibio]

LGTM. Agree with using logfmt (to start at least). Addresses all of my concerns in #1038 (comment)